It is well known to utilize liquid crystal displays which operate either in a reflective or in a transmissive mode. In the former case, incident light entering the front of the display passes through the liquid crystal solution to a reflective element positioned behind the cell. The light is reflected back through the display to the viewer. In transflective devices, the display may be operated in both the reflective and the transmissive mode. To this end, a light source is positioned behind the reflective element. In the transmissive mode the light source is energized to transmit light from the back of the cell to the front.
In both reflective and transflective liquid crystal devices it may be desirable to incorporate a resistive heating element in those applications where the display is exposed to cold temperature. The resistive heating element and a temperature sensing element such as a thermistor, when positioned in the vicinity of the display, form a combination to energize the heating element whenever the temperature falls below a predetermined trip point. However, when the resistive heating element is incorporated directly onto the liquid crystal cell, as for example by depositing a resistive coating on the rear surface of the LCD back glass, a problem is encountered when the cell is illuminated in the reflective mode because the ambient light from the front of the cell must pass through the resistive coating twice resulting in both a significant reduction in brightness and in a potential color change. This is due to the fact that typical resistive coatings both atttenuate the transmitted light as well as cause a small color shift.
Accordingly, it is highly desirable to provide a heated liquid crystal display in which the heating element is positioned so as not to have any deleterious effects on brightness and color when the cell is operated in the reflective mode. Furthermore, in many applications, it is highly desirable to utilize a design configuration which will minimize the amount of power necessary to heat the liquid crystal display to its minimum operating temperature.